As the data areal density in hard disk drives (HDD) continuously increases because of technology improvements, the magnetoresistive (MR) sensor that is used as the read-back element in HDD is required to have increasingly better spacial resolution while maintaining a reasonable signal-to-noise ratio (SNR). The sensor is a critical component in which different magnetic states are detected by passing a sense current through the sensor and monitoring a resistance change. A common tunneling magnetoresistive (TMR) configuration includes two ferromagnetic layers that are separated by a non-magnetic spacer (tunnel barrier) in the sensor stack where the tunnel barrier is typically comprised of one or more metal oxides, metal oxynitrides, or metal nitrides. One of the ferromagnetic layers is a pinned layer wherein the magnetization direction is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) pinning layer. The second ferromagnetic layer is a free layer wherein the magnetization vector can rotate in response to external magnetic fields (such as a bit field from a magnetic medium track) and is aligned either parallel or anti-parallel to the magnetization in the pinned layer.
Referring to FIG. 1a, a portion of a conventional read head 90 is shown wherein a sensor element 4 is formed between a top shield 1 and bottom shield 2, and between hard bias structures 3 that are positioned on opposite sides of the sensor. Hard bias structures 3 have a longitudinal magnetization 42 to provide a biasing magnetic field on the sides of the sensor to orientate the free layer magnetization 43 (FIG. 1b) in the y-axis direction or in-plane direction in the absence of an external field. There is an insulation layer 20 to separate the sensor from the hard bias structures. The thickness of the sensor element is also referred to as the reader shield (shield to shield) spacing (RSS) 41. As sensor size becomes smaller in a cross-track direction to achieve higher areal density, it is critical to also reduce the RSS spacing (down-track direction) in order to improve bit error rate (BER).
In FIG. 1b, a conventional sensor element 4 is shown with a bottom spin valve configuration that has a seed layer 21, AFM layer 8, SyAP layer 25, tunnel barrier 10, free layer 5 having a magnetization direction 43, and a capping layer 11 that are sequentially formed on the bottom shield (not shown). The SyAP layer comprises an outer pinned layer AP2 7, a middle antiferromagnetic coupling layer 9, and an inner pinned layer AP1 6. As a result, the AP1 and AP2 pinned layers can deflect with a very large external field and thus the SyAP magnetization (not shown) is considerably less likely to be flipped (switched) than the free layer magnetization. In this case, the resistance of the sensor changes based on the relative alignment between the magnetization of AP1 and free layers. Sensor sidewalls 100 are aligned at an angle a with respect to a bottom surface 8b of the AFM layer and may vary from sloped (<90 degrees) to essentially vertical (90 degrees) depending on the nature of the etching process used to form the sidewalls.
Current efforts to further increase areal data density involve developing a greater data linear density along a down-track (z-axis) direction and a higher track density along the cross-track (y-axis) direction. The AFM layer, which provides bias to the pinned layer magnetization and high temperature stability, is generally one of the thickest layers in the sensor stack. Therefore, it is difficult to reduce RSS spacing without modifying the AFM design.
One skilled in the art recognizes that reducing RSS spacing 41 in FIG. 1a usually means the thickness of hard bias structure 3 must decrease accordingly. As a result, a thinner hard bias structure may lead to a weaker pinning field on the edges of free layer 5 (FIG. 1b) and thereby yield a less stable sensor 4. Meanwhile, magneto-static coupling between the hard bias structure and top shield 1 may become greater as RSS spacing decreases which can easily cause a rotation of hard bias magnetization 42 away from a longitudinal direction in the proximity of free layer. Thus, modification of the AFM layer and longitudinal biasing structure around the sensor must be carefully designed in order to avoid degrading the desired properties of the sensor stack layers and longitudinal biasing structure.
An improved read head design with reduced shield to shield spacing is needed that avoids compromising sensor and longitudinal biasing structure properties while improving pinned layer flip robustness to maintain the correct magnetization direction in the presence of external magnetic fields.